Rubber-based pressure-sensitive adhesive compositions generally have no adherend selectivity and can show stable adhesive strength in application to all kinds of adherend surfaces. Even in application to rough surfaces having irregularities, satisfactory adhesive strength is obtained by increasing the thickness of the adhesive layer because the increase in adhesive layer thickness enables the pressure-sensitive adhesive to conform to the surface irregularities and have an enlarged adhesion area.
Furthermore, solid type pressure-sensitive adhesive compositions can be handled as solventless nonaqueous pressure-sensitive adhesive compositions which contain neither any organic solvent causative of environmental pollution nor water, which necessitates much energy for drying.
Such rubber-based pressure-sensitive adhesive compositions have hither to been prepared in the following manner. A rubbery polymer is used as a main ingredient, and a tackifier is mixed therewith. A filler, e.g., a fine calcium carbonate powder, and other ingredients including a softener such as an oil and an antioxidant are usually mixed therewith in appropriate amounts. This mixture is kneaded with a kneader, Banbury mixer, mixing rolls, or the like with heating to thereby obtain a pressure-sensitive adhesive composition which is solid at ordinary temperature.
This pressure-sensitive adhesive composition is applied usually with a calender or extruder to a substrate in a thickness of generally from 0.1 mm to about several millimeters to produce a pressure-sensitive adhesive sheet or tape suitable for adhesion to rough surfaces having irregularities.
Incidentally, a pressure-sensitive adhesive sheet is required to have adhesive strength which is the ability to conform to adherends and tenaciously adhere thereto. The temperature dependence of this adhesive strength also is an important property. In particular, the tapes to be used outdoors in, e.g., building/civil engineering works are required to show stable adhesive strength regardless of whether they are used in the cold winter season or in the hot summer season.
A technique effective in securing low-temperature adhesiveness is to incorporate a softener such as an oil to prevent the pressure-sensitive adhesive from becoming rigid. Incorporation of a large amount of a softener enables the pressure-sensitive adhesive to be soft even at low temperatures and have improved adhesiveness to rough surfaces. However, since the softener itself does not have the property of imparting tackiness, there are limitations on adhesiveness improvement. In addition, the corporation of a softener disadvantageously makes the pressure-sensitive adhesive too soft at high temperatures and, hence, is apt to result in a decrease in cohesive force.
As described above, low-temperature adhesiveness and high-temperature cohesive force are inconsistent with each other, and it is not easy to balance these two properties with each other by the incorporation of a softener. Namely, compositions suitable for an improvement in high-temperature cohesive force show poor low-temperature adhesiveness to rough surfaces and unavoidably suffer an adhesion failure in which application at low temperatures in winter results in adhesive layer separation. This has been a reason for an obstacle to the use of pressure-sensitive adhesive tapes as a substitute for caulking.
On the other hand, compositions suitable for an improvement in low-temperature adhesiveness show poor cohesive force at high temperatures in summer and, hence, are also apt to suffer an adhesion failure. Moreover, these compositions are apt to have a problem concerning workability that the pressure-sensitive adhesive sticks to the hands of workers.
It is known that like ordinary rubber products, rubber-based pressure-sensitive adhesive compositions of the solid type can be made to have enhanced cohesive force and retain high-temperature cohesive force even when the amount of the softener incorporated therein is increased, by crosslinking the rubber with a crosslinking agent. A technique for evenly and moderately conducting the crosslinking treatment is also known which comprises adding a tackifier to a rubbery polymer and crosslinking the polymer with an isocyanate crosslinking agent to thereby enhance cohesive force while maintaining adhesive strength (see JP-A-2001-181596 (pages 2-4)).